1. Field of the Invention
The present invention relates to an optical recording medium drive for optically recording, reproducing or erasing information onto or from an optical recording medium, such as an optical disk or an optical card. In particular, the present invention relates to an improvement of an optical head capable of reducing an optical-axis displacement due to the temperature change in an optical component, such as lens, or a housing thereof.
2. Related Art
An optical memory technology using an optical disk with pit patterns is increasingly applied to high-density high-capacity recording mediums, such as digital audio disks, videodisks, document file disks and data files. In the optical memory technology, information is recorded/reproduced onto/from the optical disk through a finely narrowed light beam with a high degree of accuracy and reliability. These recording/reproducing operations are entirely dependent on an associated optical system, and therefore it is critical to reduce the variation in characteristics of the optical system, particularly, due to temperature change.
An optical head is constructed by mounting various optical components, such as a light source, a photodetector, a half mirror and lenses, on a given frame. These optical components are required to be accurately mounted in their correct positions without causing the displacement in optical axis and focal position.
There have been known a lens holding structure for an optical pickup apparatus as disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 5-323166 and 10-334472. As shown in FIGS. 10A and 10B, a lens holding structure disclosed in the Japanese Patent Laid-Open Publication No. 10-334472 comprises an approximately columnar lens frame 124 for holding a collimator lens 113. The lens frame 124 is formed with a hollow portion to provide an optical path having an optical axis O aligned with the central axis of the lens frame 124.
As shown in FIG. 10B, the hollow portion of the lens frame 124 has one open end formed into a reverse-tapered larger-diameter shape allowing a semiconductor laser to be attached thereto, and the other open end formed into a larger-diameter shape with a short tubular inner peripheral surface 124a. The collimator lens 113 is received and held in the hollow portion while being surrounded by the inner peripheral surface 124a. 
The inner peripheral surface 124a has a radius slightly greater than that of the outer peripheral surface 113a of the collimator lens 113 to form a space 125 over the entire circumference between the inner peripheral surface 124a and the outer peripheral surface 113a. 
The inner surface of the lens frame 124 is formed with a projecting surface 124b having a ring shape arranged rotationally symmetrically with respect to the optical axis O. In an operation of holding the collimator lens 113 within the lens frame 124, an adhesive 116 is applied onto the projecting surface 124b, and one 113b of the lens surfaces of the collimator lens 113 is brought into contact with the projecting surface 124b and fixedly held by the adhesive 116, so that the collimator lens 113 is positioned in the optical axis direction.
While the radius of the ring-shaped projecting surface 124b may be set to extend up to the inner peripheral surface 124a of the lens frame 124, it is preferably set to be less than the radius of the outer peripheral surface 113a of the collimator lens 113 so as to prevent the adhesive 116 from running over between the inner peripheral surface 124a of the lens frame 124 and the outer peripheral surface 113a of the collimator lens 113. In addition, even if the collimator lens 113 is eccentrically fixed, the holding state between the lens surface 113b and the projecting surface 124b can be stably maintained substantially without adverse affect therefrom.
The space 125 formed over the entire circumference between the inner peripheral surface 124a of the lens frame 124 and the outer peripheral surface 113a of the collimator lens 113 can effectively prevent the thermal deformation of the lens frame 124 due to the change of ambient temperature from affecting directly on the outer peripheral surface 113a of the collimator lens 113. If the lens frame 124 is designed to assure a larger volume in the space 125, an effect of suppressing the eccentricity between the collimator lens 113 and the lens frame 124 will be reduced. Thus, in this case, it is required to use a modified bonding method capable of maintaining the effect.
While the thermal deformation of the lens frame 124 due to the change of ambient temperature is likely to affect on the collimator lens 113 through the adhesive 116, a force of the thermal deformation of the lens frame 124 acting to move the collimator lens 113 in the radial direction can be expected to be dispersed in all directions and cancelled by the bonding structure where the collimator lens 113 is bonded to the ring-shaped projecting surface 124b of the lens frame 124 through the adhesive 116 applied to the ring-shaped projecting surface 124b. More specifically, the bonded surface of the collimator lens 113 fixed to the lens frame 124 is arranged rotationally symmetrically with respect to the optical axis O because it is bonded to the ring-shaped surface 124b through the adhesive 116. Thus, when the lens frame 124 is thermally expanded due to the change of ambient temperature, the bonded surface of the collimator lens 113 receives forces of the thermal expansion of the lens frame 124 acting outward in all directions. That is, the respective regions of the bonded surface located symmetrically with respect to the optical axis O receive the forces acting thereon at substantially the same strength in the opposite directions. Therefore, the forces can be mutually cancelled.
The optical head is required to assure an adequate operation under the wide circumstance from a high temperature to a low temperature. In particular, there is a strong need for an excellent temperature characteristic in which a light to be detected is not affected by an optical-axis displacement arising from the relative displacement between a laser light source and the collimator lens 113. Specifically, if the relative displacement between a laser-emitting point and the collimator lens 113 is caused by the expansion of the collimator lens 113, the lens frame 124 holding the collimator lens 113, and/or an outer frame for holding the lens frame 124, due to the change of atmospheric temperature, a resulting optical-axis displacement will be likely to cause the displacement of a detected-light spot in a detector. Therefore, it is desired to provide an optical head capable of achieving an excellent temperature characteristic even under unstable atmospheric temperature.
While the optical pickup apparatus disclosed in the above patent publications is intended to form the adhesively fixed portion into a ring shape so as to eliminate adverse affects from thermal expansion/contraction, the direction of the optical-axis displacement due to thermal expansion/contraction is practically varied by the variation in the applied amount of the adhesive 116 and/or the variation in the bonded position, and the adverse affects from thermal expansion/contraction cannot be adequately absorbed.
In addition, the adhesive 116 in direct contact with the lens surface 113b of the collimator lens 113 causes stains in the lens 113. Furthermore, the relatively large space formed between the lens frame 124 and the collimator lens 113 causes the increase in size of the apparatus, and the complicated shape of the holder leads to increased cost and essentially requires to perform an adjustment operation with higher accuracy.